GC instrumentation MCQs With Answer

Introduction: GC instrumentation MCQs With Answer is designed specifically for M.Pharm students studying Advanced Instrumental Analysis (MPA 201T). This quiz collection focuses on gas chromatography fundamentals and instrumentation—covering columns, carrier gases, injection techniques, detectors, chromatographic theory (retention, efficiency, resolution), and GC-MS interfaces. Each question tests conceptual understanding and practical aspects encountered in pharmaceutical analysis, method development, and validation. Answers are provided to reinforce learning and aid quick revision. Work through the set to strengthen problem-solving skills and prepare for exams or laboratory method design involving GC techniques commonly used in drug analysis and quality control.

Q1. Which component of a gas chromatograph is primarily responsible for separating analytes based on interactions with the stationary phase?

• Injector
• Carrier gas supply
• Column
• Detector

Correct Answer: Column

Q2. Which type of column is most commonly used for high-resolution separations of volatile and semi-volatile compounds in pharmaceutical GC?

• Packed column
• Capillary (open tubular) column
• Ion-exchange column
• Gel permeation column

Correct Answer: Capillary (open tubular) column

Q3. A nonpolar stationary phase such as 5% phenyl methylpolysiloxane is most suitable for separating which class of analytes?

• Strongly polar compounds (e.g., amino acids)
• Highly ionic salts
• Nonpolar to moderately polar organics (e.g., hydrocarbons)
• Large biomolecules (e.g., proteins)

Correct Answer: Nonpolar to moderately polar organics (e.g., hydrocarbons)

Q4. According to the van Deemter equation, which term represents longitudinal diffusion and decreases with increasing linear velocity?

• A-term (eddy diffusion)
• B-term (longitudinal diffusion)
• C-term (resistance to mass transfer)
• D-term (detection limit)

Correct Answer: B-term (longitudinal diffusion)

Q5. For fastest separations with narrow peaks on a capillary column, which carrier gas generally allows higher optimum linear velocities and faster analysis?

• Helium
• Nitrogen
• Argon
• Hydrogen

Correct Answer: Hydrogen

Q6. Which injection technique is preferred when analyzing trace-level volatile analytes in a complex matrix to maximize sensitivity?

• Split injection
• Splitless injection
• On-column cold injection
• Syringe dilution before split injection

Correct Answer: Splitless injection

Q7. What is the correct expression for retention factor (k’) in gas chromatography?

• k’ = t0 / tR
• k’ = tR × t0
• k’ = (tR – t0) / t0
• k’ = tR – t0

Correct Answer: k’ = (tR – t0) / t0

Q8. Kovats retention index is primarily used to:

• Calculate column bleed at high temperature
• Standardize retention data by comparing to n-alkane retention times
• Measure detector sensitivity
• Determine carrier gas purity

Correct Answer: Standardize retention data by comparing to n-alkane retention times

Q9. Which statement correctly describes the detector response of a Flame Ionization Detector (FID)?

• FID provides universal response to all inorganic gases
• FID response is proportional to the number of carbon atoms in organic compounds (mass-sensitive for hydrocarbons)
• FID is selective for halogenated compounds only
• FID measures changes in thermal conductivity

Correct Answer: FID response is proportional to the number of carbon atoms in organic compounds (mass-sensitive for hydrocarbons)

Q10. Which detector offers the best selectivity for detecting electronegative species like chlorinated pesticides at trace levels?

• Thermal Conductivity Detector (TCD)
• Flame Ionization Detector (FID)
• Electron Capture Detector (ECD)
• Photoionization Detector (PID)

Correct Answer: Electron Capture Detector (ECD)

Q11. Which formula is used to estimate the number of theoretical plates (N) from peak width at baseline for a chromatographic peak?

• N = 5.54 × (tR / w1/2)^2
• N = t0 / tR
• N = tR – t0
• N = (tR × w) / 16

Correct Answer: N = 5.54 × (tR / w1/2)^2

Q12. In a capillary GC column, increasing the film thickness of the stationary phase typically causes which effect?

• Decreased retention time and lower capacity
• Increased retention time and higher capacity (larger phase ratio), often at expense of efficiency
• No change in retention but increased detector noise
• Immediate column deactivation

Correct Answer: Increased retention time and higher capacity (larger phase ratio), often at expense of efficiency

Q13. Which gas is commonly used as carrier when operating a flame ionization detector and why might it be chosen over helium?

• Argon, because it provides higher sensitivity for hydrocarbons
• Nitrogen, because it maximizes column efficiency at all flow rates
• Hydrogen, because it allows higher optimal linear velocity and faster analysis with FID compatibility
• Oxygen, because it enhances ionization in the detector

Correct Answer: Hydrogen, because it allows higher optimal linear velocity and faster analysis with FID compatibility

Q14. Which of the following best describes the principle of a thermal conductivity detector (TCD)?

• Measures ion current produced by combustion of analyte
• Measures change in thermal conductivity of the carrier gas caused by sample components
• Detects only electronegative compounds via electron capture
• Measures light absorption at a fixed UV wavelength

Correct Answer: Measures change in thermal conductivity of the carrier gas caused by sample components

Q15. In GC-MS with electron ionization, what electron energy is conventionally used to produce reproducible spectra and extensive fragmentation?

• 8 eV
• 70 eV
• 200 eV
• 0.5 eV

Correct Answer: 70 eV

Q16. Chemical ionization (CI) differs from electron ionization (EI) primarily because CI:

• Produces extensive fragmentation similar to EI
• Is a harder ionization technique giving no molecular ion
• Is a softer ionization technique that often yields [M+H]+ or adduct ions, aiding molecular weight determination
• Requires high vacuum in the GC inlet

Correct Answer: Is a softer ionization technique that often yields [M+H]+ or adduct ions, aiding molecular weight determination

Q17. In GC-MS quantitative analysis, what is the main advantage of Selected Ion Monitoring (SIM) compared with full-scan acquisition?

• Wider mass range coverage per scan
• Lower sensitivity due to fewer ions monitored
• Higher sensitivity and improved signal-to-noise for target analytes
• Eliminates need for chromatographic separation

Correct Answer: Higher sensitivity and improved signal-to-noise for target analytes

Q18. Which statement correctly describes the role of the transfer line between the GC column and the MS ion source?

• It cools the effluent to condense analytes before ionization
• It electrically charges analytes prior to MS
• It is heated and maintained above column temperature to prevent condensation and ensure efficient transfer into the MS vacuum
• It removes carrier gas before ions enter the mass analyzer

Correct Answer: It is heated and maintained above column temperature to prevent condensation and ensure efficient transfer into the MS vacuum

Q19. The quadrupole mass analyzer separates ions based on:

• Their magnetic moment in a static magnetic field
• Their time-of-flight through a drift tube
• The stability of ion trajectories under combined RF and DC electric fields as a function of m/z
• Their ability to pass through a series of slits without collision

Correct Answer: The stability of ion trajectories under combined RF and DC electric fields as a function of m/z

Q20. When performing quantitative GC analysis in pharmaceutical samples, why is the internal standard method often preferred over external calibration?

• It eliminates the need to prepare calibration standards
• It compensates for injection variability, sample loss, and detector fluctuations, improving accuracy
• It is only needed when using FID
• It prevents any matrix effects completely

Correct Answer: It compensates for injection variability, sample loss, and detector fluctuations, improving accuracy

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